Resolution in High Field Echo Planar Microscopy

Abstract

The application of echo planar imaging to NMR microscopy offers a temporal resolution unparalleled by other techniques. However, a major difficulty in imaging at the high field strengths used for microscopy is the effect of local field inhomogeneities caused by magnetic susceptibility effects. This can give rise to both image distortion and signal loss. In addition, the effect of diffusion in the presence of the large imaging gradients gives rise to a broadening of the point spread function and hence loss of true resolution. We compare the sensitivity of two techniques, MBEST and PEPI, to both of these effects. Analytic expressions for the signal in each echo of the two sequences are developed, and the point spread functions for the two techniques are calculated. Using PEPI, we have been able to produce images with an in-plane resolution of 50 μm from a single free induction decay. This technique has been extended to three dimensions allowing the generation of 643images with an isotropic resolution of 80 μm.